Electrostatic copy sheet of increased speed

ABSTRACT

AN ELECTROSTATIC COPY FORMED OF A BASE SHEET CONTAINING AN ALKYLENE GLYCOL HAVING 1-6 CARBON ATOMS AND A PHOTOCONDUCTIVE COATING OF ZINC OXIDE THEREON IS DISCLOSED. SENSITIZING DYESTUFFS IN A SOLUTION CONTAINING A MONOHYDROXY ALCOHOL ARE ADDED.

United States Patent Ofice Patented June 22,, 1971 3,586,532 ELECTROSTATIC COgSggSHEET OF INCREASED P ED Robert Freed, Chicago, Ill., assignor to A. B. Dick Company, Niles, Ill. N Drawing. Filed Apr. 8, 1968, Ser. No. 719,744 Int. Cl. G01d /06 US. Cl. 117201 8 Claims ABSTRACT OF THE DISCLOSURE An electrostatic copy sheet formed of a base sheet containing an alkylene glycol having l-6 carbon atoms and a photoconductive coating of zinc oxide thereon is disclosed. Sensitizing dyestuffs in a solution containing a monohydroxy alcohol are added.

This invention relates to electrophotography copy systems and to copy sheets having photoconductive coatings for use in electrostatic image development and it relates more particularly to the improvement in photoconductive compositions which are sensitive to visible light for use in the production of copy.

An electrostatic printing process is one capable of producing a visible image by electrostatic technique. A process of the type described is referred to in the art as the Xerox process, which makes use of a selenium drum, or Electrofax process, which makes us of a photoconductive coated paper and includes the steps of converting a light image or electrical signal into an electrostatic charge pattern on the photoconductive electrically insulating layer. The pattern is developed into a visible image with the resultant production of visible copy.

A typical electrostatic process conventionally includes the preparation of an electrophotographic recording element, for example by coating the surface of a base sheet such as paper with a photoconductive insulating material, such as zinc oxide dispersed in an electrically insulating film forming vehicle, such as a silicone resin. The recording element is subjected to corona discharge at high voltage to produce an electrostatic charge on the photoconductive surface. By focusing a light image onto the charged surface, the portions irradiated by the light rays are discharged to leave the remainder of the surface in a charged condition to define a latent electrostatic image. This image is rendered visible in a conventional manner, such as by applying a developing powder or a developer liquid whereby the pigment adheres electrostatically to the charged areas of the sheet and the image is subsequently fixed directly to the photoconductive coating as by heat, adhesive, vapor or the like. A detailed description of a typical electrostatic printing process is set forth in the Greig Pat. No. 2,874,063, issued on Feb. 17, 1959. The patent describes a process in which a sheet of paper having a coating containing relatively pure zinc oxide is charged by corona discharge to a potential of approximately 600 to 800 volts. A latent image is formed by photographic exposure and the image is developed by contacting the paper with a developer composition such as iron particles mixed with a powder of carbon pigmented resin.

An electrostatic recording element of the type described is ordinarily prepared by first mixing a finely divided photoconductor and the electrically insulating film forming vehicle with a volatile solvent such as by mixing zinc oxide and the silicone resin with toluene. The mixture is coated onto the base sheet and dried and the coated sheet is then cut to size.

One of the disadvantages in the zinc oxide normally used in such photoconductive layers is that the light sensitivity of the charged zinc oxide layer is normally at its greatest in the ultraviolet region of the spectrum, whereas the source of exposure may have its maximum output in a region of the spectrum which lies within the visible range, such as an ordinary tungsten lamp.

One solution to the problem of spectral response has been the use of photoconductive material, such as zinc oxide, dispersed in an electrically insulating film forming vehicle having incorporated therein a dyestuif which is capable of increasing the spectral response, as by absorbing radiant energy and transferring theabsorbed energy to the photoconductor. Amongst the sensitizing dyes that have been used with zinc oxide are fluorescein, eosin, erythrosin, rose bengal, malachite green, crystal violet, basic fushion, methyl green, brilliant green, methylene blue, acridine orange, alizarin red and other dye systems such as described in the following US. patents, namely: 3,132,942; 3,121,008; 3,110,591; 3,052,540; 3,051,569; 2,959,481; 3,047,384; 3,125,447; 3,128,179; 3,250,613 and 3,250,615, amongst others.

While dye sensitization improves spectral response and, to some extent, the speed of the photoconductor for charge acceptance and retention, such technique has not been particularly satisfactory since the disadvantage of strongly dyeing the zinc oxide layer offsets the sensitivity which is supplied to the photoconductive zinc oxide. Strong coloration of the photoconductive layer gives an unfavorable aesthetic efiect and is objectionable from the standpoint of contrast, image definition and the like.

Further, such dye sensitized systems are still deficient from the standpoint of the high speed required for the more recently developed high speed copying machines employing electrostatic techniques. While increased speed might be achieved by further increase in dye concentration, the corresponding increase in color contributed by the dyestuff is wholly objectionable and, even then. adequate speed is incapable of being achieved.

It is an object of this invention to produce electrostatic copy paper of greater spectral response and higher speed without corresponding increase in concentration of sensitizing dyes or discoloration of the photoconductive coating and it is a related object to produce a high speed 'Electrofax paper which finds use with high speed Electrofax or electrostatic copy machines which are being developed.

As used herein, the term speed is used to define light sensitivity of the electrophotographic coating and it is measured by the amount of light in foot candle seconds required to decay the charge to a stable residual potential. Thus the lower the numerical value, the faster the light sensitivity or speed of the coating. The term dark decay, measured in volts/ second, relates to the loss of charge of the electrophotographic coating in the dark. Another property used to measure electrophotographic coatings is charge acceptance which is measured in volts.

Electrophotographic coatings employed in present practice are formulated, as described, of photoconductive material, such as zinc oxide, of the type marketed by the New Jersey Zinc Company under the trade name Photox Zinc Oxide, or of the type marketed "by the American Zinc Company under the trade name Azo ZZZ-661 Zinc Oxide, and an insulating resinous binder such as styreneethyl acrylate, silicone resins (Silicone SR-82), modified urethane oil, modified urethaneoil-silicone resin, polyvinyl acetate, polyvinyl acetate-alkyd resin mixtures, modified polyvinyl acrylate, or other film forming insulating resin and photoconductive pigment as described in US. Patents Nos. 3,052,539; 3,121,006; 3,121,- 007; 3,132,941, and the aforementioned Greig Patent No. 2,874,063.

Dyestuffs capable of use for increasing the light sensitivity of the described electrophotographic coatings include: Brom Phenol Blue, Dibromofluorescein, Erythrosine B, Rose Bengal, Sulpho Rhodamine, Rhodamine B Extra, Eosin OJ concentrate, Auromine O concentrate, Quinaline Yellow, Brom Cresol Purple, Phloxine B, Acradine Orange, Fushin, Methylene Blue, Kiton Blue, Tartrazine concentrate, or others of the dyestuffs described in the aforementioned list of patents.

It has been found that the sensitivity and speed of electrophotographic coatings on conductive base sheets can be increased by formulation of the photoconductive coating to include a glycol as a component thereof. It has been found that such glycol is especially effective to increase the speed of electrophotographic coatings of photoconductive zinc oxide in a suitable insulating resinous binder which also embodies a sensitizing dye or dyes and that such increase in speed can be effected without increase in the concentration of sensitizing dyestuffs. In fact, a synergistic effect is indicated in that the combination of dye sensitization and glycol provide speeds incapable of being reached by use of the dystuff alone, even at higher concentrations, and that the increase in speed can be achieved even with a decrease in dye concentration. It has been found that the increase in speed is maximized when a monohydroxy alcohol is employed as the diluent for the dye blend in the coating composition formulated of the glycol, photoconductive zinc oxide, insulating resinous binder and sensitizing dyestuff.

As the glycol, use can be made of mono-, diand triethylene, propylene, butylene and up to hexylene glycols with best results being secured with propylene glycol and triethylene glycol. The amount of glycol, when calculated on the basis of a 1% solution of dyestuff or dye blend, may range from 0.05 ml. to 2.0 ml. of glycol per 3.5 ml. of the solution of dyestuff or, when calculated on the solids basis, the glycol may range from 0.05 to 2.0 ml. of glycol per 0.035 gram of dyestuff in the electrophotographic coating composition, with the preferred range being 0.4 to 0.8 ml. of glycol per 0.01 gram of dyestuif.

As previously indicated, best results are secured when the glycol is used in the combination with a dye system formed of a combination of dyestuffs, hereinafter referred to as a dye blend.

The discovery that the addition of glycol to an electrophotographic zinc oxide coating mix increases the speed of Electrofax paper is indeed unexpected and what is even more surprising is the discovery that the glycol addition works most effectively when employed in conjunction with a monohydroxy alcohol, such as methanol, ethanol, propanol, butanol, etc. as a solvent for the dyestuff.

Having described the basic concepts of the invention, the following examples are given by way of illustration, but not by way of limitation of the invention:

EXAMPLE 1 206 grams styrene-acrylate copolymer resin, solids (DeSoto Resin 15104-028) 134 grams polyethylene-orthophthalate polystyrene blend,

50% solids (DeSoto Resin E10413A) 1200 grams zinc oxide (New Jersey Zinc Co. Photox 80) 850 ml. toluene 12 cc. dye blend- 6 cc. propylene glycol EXAMPLE 2 450 grams styrene-acrylate copolymer (DeSoto Resin 45 grams polyvinyl acetatepolyacrylate blend (Neocryl resin B740) 30 grams polyvinyl acetateacrylic and alkyd resin blend (Du Pont resin Elvacite 2045) 2100 grams zinc oxide (Photox 80New Jersey Zinc Co.)

1830 ml. toluene 21 cc. dye blend 12.6 cc. propylene glycol 4 EXAMPLE 3 350 grams styrene-ethylacrylate resin, 42.4% solids (De- Soto resin E461-130) 1050 grams zinc oxide (Photox New Jersey Zinc Co.)

850 ml. toluene 10.5 ml. dye blend 5 cc. propylene glycol In the foregoing examples, the dye blend is a mixture of 0.4 gram of the sodium salt of Brom Phenol Blue, 0.4 gram of the sodium salt of fluorescein and 0.2 gram of euchrysine GGA concentrate, all dissolved in ml. of methanol. The dye blend is incorporated in an amount corresponding to 1 cc. of the solution per 100 grams of zinc oxide. The glycol is incorporated in Example 1 in an amount to correspond to /2 cc. per 100 grams of zinc oxide.

The compositions of Examples 1 to 3 are coated by conventional coating technique onto suitable base paper in coating weights of from 10-20 pounds per 3,000 square feet of surface area and then air dried or dried at elevated temperature at less than 350 F.

The speed or light decay of the electrophotographic coating produced by Example 1 measures 3.08 foot candle-seconds. The electrophotographic sheet produced by Example 2 has a light decay (speed) of 3.81 foot candle-seconds.

EXAMPLES 4 TO 8 Example 4 is the same as Example 1 but without the glycol.

Example 5 is the same as Example 1 but with the propylene glycol added in an amount to correspond to 0.05 ml. per 3.5 ml. of dye blend.

Example 6 is the same as Example 1 but with 2.0 ml. of propylene glycol per 3.5 ml. of the dye blend.

Example 7 is the same as Example 1 but with 0.05 ml. of triethylene glycol per 3.5 ml. of dye blend.

Example 8 is the same as Example 1 but with 2.0 ml. of triethylene glycol per 3.5 ml. of dye blend.

The following comparative speeds in foot candles/second were secured with the measurements made on a rotary electrometer device:

Example: Speed in foot candle-seconds 4 5.76

EXAMPLES 9 TO 15 To demonstrate the effect of the solvent on the Electrofax mix, a series of compositions were formulated with the same composition of Example 1 as a control but in which various alcohols were used in formulating the dye blend. One series was formulated without glycol and another series was formulated with 0.36 ml. of propylene glycol added per ml. of dye blend.

erates materially to increase the speed of the Electrofax paper having the zinc oxide, photoconductive, dye sensitized coating.

Examples 9 .to 15 show that an increase in speed is secured by the addition of propylene glycol with each alcohol dye blend but that the increase in speed was more favorable with the lower alcohols with best results being provided by methanol.

The use of glycol as a sensitizer in the manner described oflers a number of advantages in that glycols can be added to the dye blend or coating mix withoutin any way disturbing the stability of the coating or the coatability of the composition in fabrication of the electrophotographic element. This effectiveness of the glycols .to increase the speed of the paper is achieved without impairing dark decay, charge acceptance, light sensitivity or color of the electrophotographic coating.

It has been found that the speed of electrophotographic coatings which make use of zinc oxide as a photoconductor and an insulating resin as the binder can be increased by the addition of glycol, even in the absence of sensitizing dyestuffs, but the ultimate speed is less than the combination which includes one or more sensitizing dyes with the glycol.

It will be apparent from the foregoing that I have provided a simple, inexpensive and efficient means which operates unexpectedly to increase the speed of Electrofax or other photoconductive coatings without destroying or otherwise interfering with others of the desirable properties of the coating.

It will be understood that changes may be made in the details of construction, arrangement and operation without departing from the spirit of the invention, especially as defined in the following claims.

I claim:

1. An electrostatic copy sheet formed of a base sheet and a photoconductive coating on the surface of the base sheet formed of a composition containing an alkylene glycol containing 1-6 carbon atoms in combination with particles of a photoconductive material and insulating resinous binder.

2. An electrostatic copy sheet as claimed in claim 1 in 6 which the photoconductive material is photoconductive zinc oxide.

3. An electrostatic copy sheet as claimed in claim 1 in which the glycol is present in an amount within the range of 0.25 to 0.80 percent by weight of the coating solids.

4. An electrostatic copy sheet as claimed in claim 2 which contains in addition a sensitizing dyestufi or dyestuffs.

5. An electrostatic copy sheet as claimed in claim 3 in which the sensitizing dyestutf is introduced in solution with a monohydroxy alcohol.

6. An electrostatic copy sheet is claimed in claim 4 in which the alcohol is a lower alcohol.

7. An electrostatic copy sheet as claimed in claim 3 in which the glycol is present in an amount 'within the range of 0.05 to 3 ml. per 0.035 grams of dyestufi.

8. An electrostatic copy sheet as claimed in claim 3 in which the glycol is present in an amount within the range of 0.4 to 0.8 ml. per 0.01 gram of dyestuff.

References Cited UNITED STATES PATENTS 3,052,539 9/1952 Greig 1l7-201X 3,095,301 6/1963 Kostelec et a1. 961.7 3,107,169 10/1963 Bornarth 961.8 3,385,730 5/1968 Relph 1l7201 FOREIGN PATENTS 919,684 2/1963 Great Britain 961.7

ALFRED L. LEAVITT, Primary Examiner A. GRIMALDI, Assistant Examiner US. Cl. X.R. 

